1. Field of the Invention
This invention relates to communications methods and systems thereof, and, more particularly, to a signal transmission method for an optical network and a system thereof.
2. Description of Related Art
A peer-to-peer optical network, tree-type optical network, and ring-shaped optical network are three of the most popular optical networks between an optical line terminal and optical network units.
The peer-to-peer optical network system deploys an optical network (e.g., optical fiber network) to connect the optical line terminal to the optical network units. Such a peer-to-peer layout method ensures received signals that have well enough power, but is very expensive.
In a ring-shaped network, the optical network units are distributed in the shape of a ring, and the optical line terminal is connected to only one of the optical network units, and is connected to the remaining optical network units via a ring-shaped layout. However, if a part of the ring-shaped optical network is damaged or malfunction, the whole optical network cannot operate normally. One method is introduced to over the problem by providing two times the number of optical fibers that are deployed in the ring-shaped optical network (referring to H. Erkan et al., “Native Ethernet-Based Self-Healing WDM-PON Local Access RING Architecture: A New Direction for Supporting Simple and Efficient Resilience Capabilities”, IEEE International Conference, 2010). Since these optical fibers cannot be deployed at the same place and have a total of a great number, such a layout method is very expensive.
In a tree-type optical network, an optical line terminal is connected to a splitter, and connected via the splitter, in a tree distribution, to the optical network units. Such a tree-type layout method has a low cost. However, since the splitter may decay the power of received signals, the optical network unit may receive nothing but those signals that have better sensitivity (referring to D. Qian et al., “10-Gb/s OFDMA-PON for Delivery of Heterogeneous Services”, OFC 2008).
In general, the tree-type optical network employs Frequency Division Multiple Access (FDMA). When signals are upstreamed from the optical network unit the optical line terminal, Optical Beat Interferences (OBI) may occur. Accordingly, signal interferences are generated, and the optical line terminal cannot tell the signal transmitted from the optical network units.
In addition to the FDMA, Orthogonal Frequency Division Multiple Access-Passive Optical Network (OFDMA-PON) is developed to increase the transmission efficiency and save costs. However, the technique needs greater signal receiving power.
Another solution uses Wavelength Division Multiplexing Laser among the optical network units (referring “10 Gbps OFDMA-PON”, US Patent Application Publication No. 2009/0097852 to Dayou QIAN et al.). However, the temperature of the WDM Laser needs to be controlled, in order to maintain accurate wavelength. Besides, the wavelengths have to be allocated to certain optical network unit in advance. The solution lacks flexibility.
A Colorless architecture (referring to Cvijetic, N. et al., “Orthogonal Frequency Division Multiple Access PON (OFDMA-PON) for colorless upstream transmission beyond 10 Gb/s”, IEEE JOURNALS, 2010) may avoid OBI and need no WDM Laser. In this architecture, an optical filter is needed to filtering light, and an optical amplifier is also needed to increase the signal receiving power. Unfortunately, both the optical filter and the optical amplifier are very expensive, and the architecture thus has a high cost.
Since the signal transmission method for an optical network and its related system has the problems of suffering signal interferences, receiving nothing but signals that have greater power, and costing a lot of money, providing a signal transmission method for an optical network and a related system that may solve the above problems is becoming one of the most popular issued in the art.